Remove unneccessary "Learn more" strings.

We have a lot of places where we use "Learn more" for some link text.

In principle, this could translate differently in different locales depending on the context. But often the "Learn more" link is a stand-alone standard element, which should probably just use the same name (as Mozilla).

In practice, it is not a big deal for translators when Weblate can offer the "Learn more" translation from memory. But it can still lead to some inconsistencies. E.g. I recall that one of our locale uses a different translation of "Learn more" than Firefox, I think it was French.

We should find out:

1. Can we replace a lot of these cases with moz-support-link-text as the source of truth for "Learn more" and its translations?
2. What can we learn from Mozilla developers? Have they determined that "Learn more" is safe to translate for many contexts?
3. Does the text ever vary in our own translations, and differ from what Mozilla translators choose?

If so, we can come up with a new policy for what to do with newly created "Learn more" elements and put it in the relevant wiki page. And we can possibly drop some of our existing ones.